This invention generally relates to fiber optics. More particularly an aspect of this invention relates to a tunable chromatic dispersion compensating module (DCM) for use in fiber-optic telecommunication systems.
Chromatic dispersion is pulse spreading arising from differences in the speed that light of different wavelengths travel through a material, such as fiber optic cable. Chromatic dispersion is the variation in the propagation speed of light as a function of wavelength. Chromatic dispersion causes a distortion of the optical pulses that propagate through a fiber optic transmission line. To compensate for the chromatic dispersion in the fiber spans, chromatic dispersion compensating modules (DCMs) are placed periodically in the transmission line. Chromatic dispersion compensating modules add dispersion to the signal, which is ideally equal and opposite in sign, to counteract the dispersion accumulated in the fiber span. The pulses are then reformed to counteract and eliminate the chromatic dispersion-induced distortion within a passband of wavelengths.
In prior technologies, All-pass filters have been tried in dispersion compensation devices. FIG. 1 illustrates a basic etalon-type all-pass filter. The backside mirror has 100% reflector while the front side mirror can have any reflectivity. The term all-pass means that no fundamental sources of loss in the device exist, and thus, the theoretical amplitude response equals unity at all wavelengths. The etalon-type all-pass filter therefore only effects the phase of the light. Since dispersion is a change in the phase of the light, this type of filter is well suited to chromatic dispersion compensation.
In FIG. 1, the light travels into the basic etalon-type all-pass filter. The light is collimated and sent at normal incidence into the basic etalon-type all-pass filter. The basic etalon-type all-pass filter produces a variation in the time delay due to the resonate circulation of some wavelengths within the cavity. For wavelengths that are at resonance, the light effectively stays inside the cavity longer than for wavelengths that are off resonance. This causes a wavelength-dependent delay that produces dispersion. Light traveling out of the fiber eventually returns to the collimating lens.
FIG. 2 illustrates a chain of resonators coupling to circulators. After the light exits the collimating lens, the light recouples into the fiber where the output light can be separated from the input light with a circulator. Typically, the cascaded circulator method produces a large amount of power loss (xcx9c1.5 to 2 dB per resonator) in the light signal being compensated for chromatic dispersion.
FIG. 3 illustrates the chromatic dispersion response of a single basic etalon-type resonator. Each resonator cavity may consist of an all-pass filter from FIG. 2 and generate a wavelength delay, such as 8 ps, at a given wavelength, such as 1548.4 nm. Both the magnitude of the delay, such as 8 ps, and the affected wavelength such as 1548.4 nm may be adjustably controlled by components in the resonator cavity.
FIG. 4 illustrates an exemplary first passband of wavelengths from 1549.1 nm to 1549.3 that incur chromatic dispersion from a set of basic etalon-type resonator to counteract the chromatic dispersion that occurs when those wavelengths propagate through the fiber optic transmission system. A single pulse of light may consist of many wavelengths in a given passband entering a fiber optic transmission system. During the travels through the fiber transmission system that single pulse of light becomes multiple pulses of light slightly separated in time due to effects of chromatic dispersion. Therefore, a chromatic dispersion module""s overall goal is to delay wavelengths in a given passband enough to combine all the wavelengths in the passband into a single combine, pulse. In this example, the passband of wavelength from 1549.1 nm wavelengths to 1549.5 nm has dispersed over 80 ps. Thus, the pulse containing the 1549.3 nm wavelength trails the 1549.1 nm pulse by approximately 80 ps.
In order to counteract the chromatic dispersion induced by the fiber optic transmission system, the basic etalon-type all-pass filter produces a delay on all of the wavelengths in the passband to put the wavelengths in the same timeframe.
Various methods, apparatuses, and systems in which a chromatic dispersion compensation module includes a beam spatial orientation device to separate an optical signal into a first polarized light signal and a second polarized light signal. The second polarized light signal has the orthogonal polarization of the first polarized signal. A wavelength-dependent delay path couples to the beam spatial orientation device. A polarization rotator couples to the wavelength-dependant delay path such that the first polarized ligth signal reflects into the wavelength-dependant delay path in substantially the opposite direction of the second polarized light signal.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.